In the exploration of subsoil resources, it is customary to rely on the measurement of the vertical component of the gravity field and the vertical gradient of the same field. From an analysis of these data, it is possible to deduce information on the density distribution of the subsoil which characterizes a particular site.
On the basis of this methodology, it is also possible to obtain the mass variation of the hydrocarbons present inside a reservoir.
The idea at the basis of this technique is that as the movements of hydrocarbons inside a reservoir are correlated to density variations, they can be appreciated by means of vertical gravity gradient measurements.
It is since the thirties', in fact, that measurements of the gravimetric field gradient have been successfully used in the exploration of resources of the subsoil. Since 1936 the importance has been known of the use of the vertical gradient which, as it has a better resolution and is relatively insensitive to regional effects, often has particular structures which cannot be easily obtained from gravimetric field data.
The measurement of the vertical gravimetric field gradient can be effected by means of specific instruments called gradiometers.
Alternatively, the vertical gradient of the gravity field of a point can be measured, with good approximation, by means of the almost contemporaneous acquisition of two gravimetric measurements referring to different heights.
In this second case, before interpreting the data acquired in the field in geological terms, their reduction in terms of Bouguer anomaly is frequent, from which the undesired effects are removed and the calculation and analysis of the vertical gradient is subsequently effected.
The most important corrections to be made are the following:                Instrumental drift        Tidal correction        Latitude correction        Free Air correction        Bouguer correction        Topographic correction        
The description of these corrections is treated hereunder.
Instrumental drift: the readings of data with a gravimeter undergo time variations due to the elastic characteristics of the materials which form the instrument itself. The instrumental drift can be easily determined by repeating the measurement in the same station in different times, typically every 1-2 hours. The representation referring to Cartesian axes gives the drift curve which, for many gravimeters is of the linear type.
A definite value is subtracted with each measurement effected in subsequent stations, on the basis of the measurement time.    Tidal correction: the drift measured in reality contains the further contribution of an effect of the sea type due to moon-sun attraction (tide). The correction to be made is calculated on a theoretical basis by means of formulae which allow the quantification of this effect, such as, for example, the Longman formula.    Latitude correction: both the Earth's rotation and its equatorial swelling produce an increase in gravity with the latitude, and this must be considered when reducing the gravity data observed.    Free air correction: this is a correction used in order to consider the altitude of the measuring station.    Bouguer correction: this correction is used to consider the attraction due to the interposed masses between the measuring station and the reference surface. In 1749, Bouguer suggested that this additional attraction could have been calculated like that due to the action of an infinite horizontal plate having a thickness equal to the elevation from sea level of the measuring station.    Topographic correction: The approximation of the plate may be unsatisfactory in an area with an articulated topographic trend.
Under these conditions, it is appropriate to add a correction in order to consider the masses above the plate and those whose contribution has been erroneously subtracted in the Bouguer correction.
After the reductions listed, the vertical gradient of the field is calculated as described hereunder.
Variations in density in the subsoil with time can be measured and monitored from the measurements of the vertical gravity field gradient.
This method is already in use for measuring and monitoring water layers and geothermic fields.